The invention relates to new pyridazinone derivatives of the formula I ##STR2## wherein R is F, Cl, Br, I or R.sup.1 R.sup.2 N,
R.sup.1 and R.sup.2 are each H, alkyl or benzyl, PA0 R.sup.3, R.sup.4 and R.sup.5 are each H or alkyl and PA0 the alkyl groups each contain 1-4 C atoms, but wherein it is only possible for R to be benzylamino, R.sup.3 to be CH.sub.3 and R.sup.4 to be H at the same time if R.sup.5 is alkyl, PA0 R.sup.1 and R.sup.2 are each independently H, C.sub.1-4 -alkyl or benzyl, PA0 R.sup.3, R.sup.4 and R.sup.5 are each independently H or C.sub.1-4 -alkyl, PA0 in Ia R.sup.3 is alkyl having 1-4 C atoms; PA0 in Ib R.sup.3 is CH.sub.3 ; PA0 in Ic PA0 in Id PA0 in Id R is F, Cl, Br or I; PA0 in Ie R is R.sup.1 R.sup.2 N; PA0 in If PA0 in Ig
and also to salts thereof.
A similar compound "5-methyl-6-(3-nitro-4-benzylaminophenyl)-4,5-dihydro-2H-pyridazin-3-one" (corresponding to I with R being benzylamino, R.sup.3 being CH.sub.3 and R.sup.4 and R.sup.5 being H) is mentioned in German Offenlegungsschrift No. 2,922,336, but a process for its preparation is not mentioned therein.